In 1903 Pope St. Pius X inaugurated a century of
emphasis on "actual participation" at Mass -- the actuosaparticipatio
famously reiterated by Vatican II -- when he wrote:

"The
Holy Mass is a prayer itself, even the highest prayer that exists. It is
the Sacrifice, dedicated by our Redeemer at the Cross, and repeated every
day on the Altar. If you wish to
hear Mass as it should be heard, you must follow with eye, heart, and mouth
all that happens at the Altar. Further, you must pray with the Priest the
holy words said by him. You have to associate your heart with the holy
feelings which are contained in these words and in this manner you ought to
follow all that happens on the Altar. When
acting in this way, you have prayed Holy Mass."

With a missal in hand as you view a telecast of a
Mass, you can pray the Mass in this same way. As you would if physically present
(though not, of course, satisfying any Sunday or holy day obligation to
attend Mass).

Spiritual
Communion

When we view a Mass that we cannot attend, we can
make what is known as a "spiritual Communion", an act expressing
what was described by St. Thomas Aquinas as "an ardent desire to receive
Jesus in the Most Holy Sacrament and in lovingly embracing Him." In a
spiritual Communion, we, with contrite, humble hearts, ask our Lord to come
to us in the same way He would if we were able to receive the Sacrament
physically.

Some Catholic authorities teach that the graces received
by spiritual communion can equal (or in some cases even exceed) those
received from the actual Sacrament by someone who is physically present.
Though, of course, the Sacrament itself is inherently greater, our
disposition toward the Sacrament can greatly affect the extent to which we
actually receive its fruits in our soul.

How to
Receive Spiritual Communion

St. Leonard of Port-Maurice offers this advice for receiving Spiritual
Communion:

In
order to facilitate a practice of such great excellence, ponder what I have
to say. When the priest is about to give himself Communion in holy Mass, do
you, keeping composed externally and internally, excite in your heart an act
of true contrition, and humbly striking your breast, in token that you
acknowledge yourself unworthy of so great a grace, make all those acts of
love, of self-surrender, of humility, and the rest, which you are
accustomed to make when you communicate sacramentally,
and then desire with a lively longing to receive your good Jesus, veiled in
the sacrament for your benefit. And to kindle your devotion, imagine that
most holy Mary, or some saint, your holy advocate, is holding forth to you
the sacred particle; figure yourself receiving it, and then, embracing
Jesus in your heart, reply to Him, over and over again, with interior words
prompted by love: “Come, Jesus, my Beloved, come within this my poor heart;
come and satiate my desires; come and sanctify my soul; come, most sweet
Jesus, come!” This said, be still; contemplate your good God within you,
and, as if you really had communicated, adore Him, thank Him, and perform
all those interior acts to which you are accustomed after sacramental
Communion.

You can make an act of
spiritual communion (as below) at the same time as those present at the
Mass are receiving holy communion sacramentally.
Though you can use your own words, many people say those recommended by St. Alphonus Liguori:

Act of Spiritual Communion

My Jesus, I believe that Thou art truly present
in the Most Blessed Sacrament.
I love Thee above all things, and I desire to receive Thee into my soul.
Since I cannot at this moment receive Thee sacramentally,
come at least spiritually into my heart.

I embrace Thee as being already there, and
unite myself wholly to Thee.

Never, never permit me to
be separated from Thee.

Jesus, my sweet love, wound and inflame this heart of mine, so that it may be always and
all on fire for Thee.

Amen.

Just as many people say the Anima Christi immediately after
sacramental Communion, so too can you say it after spiritual communion.

The Anima Christi

Soul of Christ, sanctify me.
Body of Christ, save me.
Blood of Christ, inebriate me.
Water from Christ's side, wash me.
Passion of Christ, strengthen me.
O good Jesus, hear me.
Within Thy wounds hide me.
Suffer me not to be separated from Thee.
From the malicious enemy defend me.
In the hour of my death call me.
And bid me come unto Thee.
That I may praise Thee with Thy saints and with Thy angels.
Forever and ever. Amen

Or the following prayer of St. Thomas Aquinas while the celebrant is
completing the ablutions:

Prayer of Thanksgiving after Holy Communion

Lord, Father all-powerful and ever-living God,
I thank You, for even though I am a sinner, your unprofitable servant, not
because of my worth but in the kindness of your mercy, You have fed me with
the Precious Body & Blood of Your Son,
our Lord Jesus Christ. I pray that this Holy Communion may not bring me
condemnation and punishment but forgiveness and salvation.

May it be a helmet of faith and a shield of
good will. May it purify me from evil ways and put
an end to my evil passions. May it bring me charity and patience, humility
and obedience, and growth in the power to do good.
May it be my strong defense against all my enemies, visible and invisible,
and the perfect calming of all my evil impulses, bodily and spiritual. May it unite me more closely to you, the One
true God, and lead me safely through death to everlasting happiness with
You.

And I pray that You will lead me, a sinner, to
the banquet where you, with Your Son and holy Spirit, are true and perfect
light, total fulfillment, everlasting joy, gladness without end, and
perfect happiness to your saints. grant this
through Christ our Lord, Amen.